Neisseria gonorrhoeae MS11 contains 11 genes encoding opacity proteins (Opa), a family of outer membrane proteins, which contribute to colony opacity and are believed to be involved in a number of functions including interaction with the host cells. Opa proteins are very similar to each other with the exception of one semi-variable (SV) and two hypervariable (HV) regions. Particular Opa proteins confer on N. gonorrhoeae the ability to adhere to tissue culture cells of human or non-human origin and to be taken up by human conjunctiva (Chang) cells. Other Opa proteins appear to be involved in the interaction of N. gonorrhoeae with human polymorphonuclear leukocytes (PMN's). In order to determine functional domains of the Opa proteins of N. gonorrhoeae, chimeric opaA, B, and C genes of N. gonorrhoeae MS11 were constructed. opaA, B, and C were selected from the repertoire of strain MS11 for the following reasons: OpaA mediates the adherence and uptake of N. gonorrhoeae by heparan sulfate-expressing epithelial cells, but this Opa renders N.gonorrhoeae non-reactive with PMN's in vitro; OpaC expressing N. gonorrhoeae adhere to both Chang cells and PMN's but are only internalized by PMN's and not Chang cells; OpaB expressing N. gonorrhoeae do not interact with Chang cells but do adhere to and are internalized by PMN's, thereby resembling N. gonorrhoeae that express the other Opa proteins (D, E, F, etc.) of this strain's repertoire. That portion of each opa gene that encodes its HV2 hypervariable region was replaced by the analogous region of the other two opa genes. In this way, six chimeric recombinant opa genes were constructed (AB2, AC2, BA2, BC2, CA2, and CB2). Mutants of opaA were also constructed such that the HV1 (deltaHV1) and HV2 (deltaHV2) regions of OpaA were deleted. Chimeric and deletion opa genes were expressed from a plasmid under control of a synthetic opa promoter in a strain of N. gonorrhoeae MS11 in which the opaA gene had been mutated by insertion of a chloramphenicol resistance marker. N. gonorrhoeae MS11 expressing chimeric and deletion opa genes were examined in several assays for Opa structure-function correlations. These include binding assays with heparin and the ectodomain of Chang cell surface heparan sulfate proteoglycans, and adherence to, and internalization by tissue culture cells.